1. Field of the Invention
The present invention relates to a toner for developing electric latent images or magnetic latent images in electrophotographic methods, electrostatic printing methods or magnetic recording methods, and more particularly, to a toner suitable for fixing the developed images to a support by means of heat roller fixation.
2. Description of the Prior Art
Heretofore, there have been known various electrophotographic processes such as those disclosed in U.S. Pat. Nos. 2,297,691, 2,825,814, 3,220,324 and 3,220,831, British Pat. Nos. 1,165,406 and 1,165,405 and the like. In general, an electrophotographic method comprises utilizing a photoconductive material, forming electric latent images on a photosensitive member by one of various means, developing said latent images with a toner, if desired, transferring the developed toner images to an image receiving member such as paper, and fixing the toner images by means of heat, pressure or solvent vapor.
In addition, there are known various magnetic recording methods for forming magnetic latent images and developing with a magnetic toner.
In connection with these methods, various methods and devices have been developed for fixing toner images onto paper and the like.
The most common method is the so-called heat-roll fixing system where both heat and pressure are simultaneously applied. This method comprises bringing an image receiving sheet bearing toner images into contact with a heated roller to fix the toner images to the image receiving sheet. However, when such fixing system is employed together with a conventional toner, there occur troubles such as offset and the like. Offset is an undesirable phenomenon that a part of toner images carried on an image receiving sheet is transferred onto the surface of the roller.
As mentioned in U.S. Pat. No. 3,941,898, such offset phenomenon is liable to occur when a low molecular weight polymer is used. Therefore, the offset phenomenon seems to be prevented by using a crosslinked polymer as proposed in said U.S. patent. However, the present inventors have found that crosslinked polymers are not always good for such purpose, and in particular, in case of magnetic toners the fixing temperature is raised and the anti-offset property is lowered. In other words, when, for example, various styrene-butyl acrylate copolymers different in the degree of crosslinking were prepared by using divinylbenzene as a crosslinking agent and toners were made therefrom and the fixing property and the anti-offset property were measured, it was found that the higher the degree of crosslinking, the less the offset occurs, but the higher the fixing temperature. The fixing temperature range of the crosslinked polymers is clearly broader than that of the non-crosslinked ones, but the former is not yet at a practically satisfactory level when it is used for a fixing roller having a surface of low releasing property as the result of fatigue.
The surface layer of a roller contacting toner images is usually composed of silicone rubber of fluorine containing resin which is of good releasing property. Some rollers are coated with a releasing oil such as silicone oil and the like so as to prevent offset on the roller surface and fatigue of the roller surface. Such oil coating systems, however, make the fixing device complicated since an oil coating means should be provided, and moreover, the oil evaporated by heat is detestable to the user.
Therefore, such oil coating is not a desirable countermeasure to offset, and at present, researches are directed to development of a toner having wide fixing temperature range and good anti-offset property.
Naturally the toners should have simultaneously excellent anti-blocking property, developing property, cleaning property and the like as well as excellent fixability. Conventional toners have one or more of the following drawbacks. For example, most toners which are easily melted by heat are liable to cake or agglomerate during storage or in a copying machine. Most toners are adversely affected by emvironmental humidity change and therefore the triboelectric property and fluidity become poor. Further in most cases, when toners are continuously and repeatedly used for development, the collision between toner particles and carrier particles and contact of these particles with the surface of a photosensitive surface cause deterioration of the toner particles, carrier particles and photosensitive plate resulting in change of the image density thus obtained, increase in the background density and lowering of the quality of copy. Furthermore, if increase in density of copied images is contemplated by increasing the amount of toner attached to the surface of a photosensitive plate having latent images, usually the background density increases and so-called fog is formed.
In view of the foregoing, toners having excellent toner characteristics and suitable for heat-roller fixation are desired at present.